I would probably make it more rat like, mainly because I wouldn't have known it was a mutant rat if I didn't read the title. Maybe add the rat teeth or the ears of the rat. You need to know what it is mutated from to be truely afraid of it(you know if you want people to fear it)

Valid, perhaps making it more rat-like is a fine thought. Though mutation can be extremely severe to a point where it cannot be identified. And on personal perspective, I would say it is far more frightening if you cannot identify what your witnessing.

You do have a point, though maybe adding some extra disfigurements would make it all the more alluring. Possibly boils. Either way, it's scary when it's about to kill/attack you. BTW That is an awesome peice of art, I forgot to add that to my comment earlier.